This invention relates to a wire dot printer head which drives a print wire by biasing a leaf spring and releasing the bias force of the leaf spring.
Conventionally, in wire dot printer heads of this kind, an armature has been welded at the position adjacent to the free end portion of a leaf spring by spot welding, laser welding or the like. A carbon tool steel which is superior in elasticity and fatigue resistance, such as JIS steels SK-3 to SK-5 (consisting of 0.80-1.10% C, not more than 0.35% Si, not more than 0.50% Mn, not more than 0.030% P, not more than 0.030% S, and the balance Fe), has been used as the material for a leaf spring, and a low carbon steel which has good magnetic properties has been used as a material for an armature. However, there have been caused such drawbacks that fusion due to welding causes segregation of the supersaturated carbons in the leaf spring material of a carbon tool steel, and that a change in volume caused by the change in crystal structure due to the fusion causes micro-cracking in the weld, both of which are remarkably deleterious regarding fatigue life time relating to resistance against the fatigue caused by the bending force occurring on the weld metal every time a print wire is driven to effect printing on a printing paper and the reduction of which resistance causes breakage of the leaf spring and trouble in the print head. That is, when the print wire is projected by means of the resilient force of the leaf spring, the print wire collides with carbon sheet, printing paper and a platen, so that impact results in bending of the leaf spring which bending causes the leaf spring to oscillate with fulcrums at the fixed end of the leaf spring and at the welded portion. The fixed end of the leaf spring is not damaged by this oscillation because no degradation is caused in the fixed end. But, if the welded portion has any micro-cracks or segregation of carbon, repeated oscillation causes fatigue on the leaf spring and then causes cracks of the spring. Thus, there are required regarding the properties of wire dot printer head not only such matter that the leaf spring can be firmly welded to an armature but also such matter that the welded portion has large fatigue resistance.
As to a material for leaf spring of a wire dot printer head, Japanese Patent Laid-Open Publication No. 3952/1983 has disclosed that 17-7 precipitation-hardened stainless steel plate consisting of 16.4 to 17.5 wt % Cr, 6.5 to 7.5 wt % Ni, 0.9 to 1.4 wt % Al, 0.06 to 0.08 wt % C, 0.4 to 0.9 wt % Mn, 0.15 to 0.64 wt % Si and the balance being Fe has resilient force and fatigue resistance similar to conventional carbon tool steel (JIS steels SK-3 to SK-5) and has better resistance to heat deterioration caused due to welding. Although the 17-7 precipitation-hardened type leaf spring shows some excellent properties, it is inferior to the carbon tool steel in the matter of resistance to repeated impact.